Modifying capacity effect



May 28, 1940. K, oam 2,202,506

- MODIFYING CAPACITY EFFECT Filed April 16, 1937 2 Sheets-Sheet 1 OUTPUT y 8, 1940. H. K. ROBIN 2,202,506

MODIFYING CAPACITY EFFECT Filed April 16, 1937 2 Sheets-Sheet 2 Flg. //.7.

Patented May 28, 1940 UNITED STATES PATENT OFFICE MODIFYING CAPACITY EFFECT Application April 16, 1937, Serial No. 137,259 In Great Britain May 15, 1936 5 Claims.

This invention relates to methods of and means for modifying capacity effects in electrical apparatus and is particularly applicable in reducing the effective capacity of thermionic valves and high frequency transformers.

The invention will first be discussed in relation to thermionic amplifiers in which the input voltage is stepped up by means of a transformer. In such an amplifier the frequency range for satisfactory amplification is limited (a) by the stray capacities associated with the transformer secondary winding and (b) by the grid-toground capacity of the valve as well as by other factors. In order to reduce the deleterious effects of these capacities it is proposed substantially to eliminate or reduce the currents due to these capacities. At any given frequency, the current flowing in a condenser is proportional both to the magnitude of the condenser and to the applied alternating voltage and according to the invention it is proposed to reduce preferably to zero the applied voltage thus reducing the current flow.

Accordingly in the case under discussion the capacitative currents from the transformer secondary winding or the valve grid are reduced or eliminated by the provision of a screen for the winding or grid, such screen having a potential applied to it from an external source equal to the potential of the winding or grid. Further, the

capacity between the screen and earth is arranged to appear in a circuit separate from the input circuit of the valve so that the latter circuit is not adversely affected thereby. If the potential in the element having stray capacity increases along the element as in the case of a transformer winding, it is desirable to provide a similar voltage rise along the screen as hereinafter explained in more detail.

It will be apparent that the method is applicable to reduce the elfective capacity in other apparatus and circuits having an element in undesired capacitative relation to other parts of the system and subject to alternating potentials of sufficiently high frequency to make the capacitative current appreciable.

The invention in one aspect thereof accordingly resides in the method of reducing capacity eifects in electrical apparatus such as a transformer or thermionic valve having an element in undesired capacitative relation to other parts of the apparatus and subject to alternating voltages which consists in providing a screen for said element and applying to said screen a potential equal or proportional to that of said element, the potential being derived and applied in such a manner that the capacity between the screen and said other parts of the apparatus appears in a circuit where its presence is not deleterious.

According to another aspect, the invention resides in a thermionic valve arrangement comprising input and output circuits and means for reducing the effect of capacity between earth and one of the elements of the input circuit comprising a screen for said element fed from the output circuit to have a potential substantially equal or proportional to that of the element and so arranged and connected that the capacity between the screen and earth appears in the output circuit.

An embodiment of the invention in which the harmful effect of certain stray capacities in a push-pull amplifier is substantially eliminated will now be described with reference to the accompanying drawings.

In the drawings, Fig. 1 is a circuit diagram of going ohm transmission line is connected over a 2 to l transformer 3 the primary winding of which is connected in the cathode leads of the valves common to both anode-cathode and control-grid-cathode circuits. Means i is provided for producing a bias voltage for the control grids of the valves. Connections to the heaters and to the auxiliary grids in the valves are made in known manner. In order to obtain a satisfactory balance more readily, the electrical mid-point of the secondary of transformer I2 is obtained by means of two high resistances it. In such an amplifier, potential differences appearing across the output impedance are fed back to the grid circuit'in phase opposition to incoming grid Voltages and operate to reduce the harmonic content of the output. For maximum power output the output impedance for each valve is given by where g is the mutual conductance of the valve and the output voltage V2 across the primary of transformer 3 bears a ratio to the input voltage V1 across the secondary of transformer 2 of the order of 1 to 2. That is to say a reduction of voltage occurs but as the output voltage is working into a low output load the power again is considerable. This amplifier circuit is suitable for the amplification of a very wide range of frequencies.

The input transformer 2 is of the toroidal type and has a low impedance winding consisting of five turns which may be wound immediately next to the core and a secondary winding of one hundred and twenty turns wound in a single layer as shown digrammatically in Fig. 2. The capacity between the secondary winding and the core is excessive and means in accordance with the invention is therefore provided to reduce the effects of this capacity. This means comprises a non-inductive resistive screen 5v interposed between the core and the secondary winding and connected to serve as anode resistances for the valves A and B. The Value of resistance is so chosen that the alternating potential fall along the length of the screen is identical with the potential fall in the secondary winding so that at every point along the secondary winding there is a portion of screen of the same potential and capacity currents between the winding and the core and between the winding and the screen are reduced substantially to zero. The capacity between the screen and the core appears in the output circuit where its presence is permissible since the output voltage from cathode to ground 'is half the input voltage from grid to ground and the output impedance is equal to the resistance necessary in the anode circuit to provide a voltage equal to the grid voltage is ohms Where g is the mutual conductance of the valve. Each half of the resistive screen is therefore given this resistance and as the anode voltage of each valve is out of phase with the grid voltage of that valve the section of the screen adjacent the secondary terminal connected to the grid of one valve is connected to the anode of the other valve as shown. The voltages in adjacent parts of the winding and the screen are then identical in magnitude and phase. As the voltage fed back is equal to and not greater than the input voltage, no building up of oscillation occurs.

A practical construction of the transformer is shown in Figs. 2 to 5. The ring-shaped core 6 is provided with two annular discs I of Bakelite or similar insulating material having a resistance material 8 sprayed on one side and provided with suitable terminal tags at 9 and iii. The coating of resistance material is interrupted between the tags 9 which form the ends of the screen for connection to the anodes of the valves. Further insulation I2 is added and then the toroidal winding i3 is applied.

With the single layer winding, very satisfactory elimination of stray capacity efiects can be secured. If a multilayer winding is employed only an approximate correction is obtained.

In the case of a transformer, the stray capacity is distributed over the winding and in the construction described the correcting efiect is also distributed along the winding. An approximate correction can however be obtained by considering the stray capacity to be lumped at the ends of or at points in the winding and providing screens of suitable potential at those ends or points. In the case of a thermionic valve the capacity between the grid and another electrode can be rendered ineffective by a screen of high conductivity interposed between the electrodes concerned and suitably connected to the anode circuit of the valve.

An amplifier arrangement including means for reducing the effects of capacity between the grid of a valve and other electrodes according to the invention is shown in Fig. 6.

The amplifier comprises two stages each similar to that of Fig. 1, each stage including a pair of high slope output pentodes. The suppressor grids of the valves, connected to ground, have been omitted for simplicity. Resistive screens are provided in each transformer as previously described; they are indicated in the drawings at E5, it, ill and I8 and to simplify the drawings their actual relation to the transformers is not shown. As has already been explained, the anode resistance, say E5, of one valve is such that the alternating voltage drop therein is equal to the input voltage of the opposite valve (B) and the anode end of the resistance is of the same alternating potential as the grid end of the screened transformer winding. The anode of one valve of each push-pull pair is thus of the same potential with regard to alternating currents as the grid of the other valve of the pair and the phase is the same. In order therefore to reduce the inherent anode-control grid capacity efiect the screen grid of valve A is connected directly to the anode of valve B and the screen grid of valve B directly to the anode of valve B as shown. Each control grid is in this manner provided with a screen of the same potential so that no capacitative current will flow between the grid and the screen. The capacity between the screen and the anode appears in the output circuit where its presence is permissible.

If desired the screen grid in each valve may be arranged entirely to surround the control grid so that the capacity effects between the control grid and all other adjacent electrodes are eliminated.

The push-pull arrangement of valves permits a screen voltage to be obtained in a simple manner which is of suitable phase. In the case of a single valve amplifier other means such as a small auto-transformer in the anode circuit must be provided so that the phase of the screen matches the phase of the input circuit element which is to be screened.

In order to balance the parts of the amplifier particularly for noise balance, the electrical midpoints of the incoming and outgoing lines are connected to ground, the former mid-point being obtained by balanced condensers 20 preferably of the single-control differential type.

What is claimed is:

1. A transformer comprising a ring shaped core, a toroidal winding on said ring-shaped core, annular discs formed of resistance material interposed between said winding and the core, said resistance material of the discs being interrupted in the neighbourhood of the ends of the winding, and connections to the resistance material provided at points on either side of the interruptions.

2. An amplifier having input and output circuits, said amplifier comprising a step-up transformer of high ratio feedinga pair of valves in,

push-pull relation, a cathode lead common to said input and output circuits, an output impedance for said valves connected in said cathode lead, a noninductive resistive screen shielding the secondary winding of the transformer, said screen being connected in the anode circuit of the valves and so dimensioned that the alternating component of the Voltage therein is substantially equal at each point along the screen to the voltage in the adjacent portion of the secondary winding.

3. A push-pull amplifier comprising a step-up transformer of high ratio feeding a pair of valves having input and output circuits, a cathode lead common to both said input and output circuits, an output impedance for said valves connected in said cathode lead, screens for the control electrodes of the valves, impedances in the anode circuits of the valves comparable in magnitude to the output impedance and a direct connection from the anode of each valve to the screen of the other valve.

4. An amplifier having input and output circuits comprising a pair of vacuum tubes in pushpull relation, a toroidal step-up transformer of high ratio having a core and primary and secondary windings in said input circuit, a screen comprising ring shaped discs of substantially uniform resistance material between said secondary winding and said core, said discs being interrupted in the neighborhood of the ends of said secondary winding, and means for connecting the interrupted ends of said disc to the output circuit of said valves so as to apply a voltage thereto, the alternating component of which is substantially equal to the voltage in the adjacent portion of said secondary winding.

5. In a circuit including a winding associated with metallic core, means for reducing the effect of capacity between the Winding and the core comprising a screen between said Winding and said core and substantially co-extensive with said winding said screen comprising a continuous sheet of resistive material, and a circuit including a source of potential separate from the circuit including the winding and arranged to pass a current through said screen such that the potential drop therein is substantially the same as that in the winding.

HAROLD KILNER ROBIN. 

